The U.S. and Israel showed signs of seeking to defuse tensions on Sunday ahead of a speech in Washington by Israeli Prime Minister Benjamin Netanyahu when he will warn against a possible nuclear deal with Iran.

Policy differences over the negotiations with Iran remained firm, however, as Netanyahu arrived in the United States on Sunday afternoon for a speech to Congress, which has imperiled ties between the two allies.

Israel fears that U.S. President Barack Obama's Iran diplomacy, with an end-of-March deadline for a framework accord, will allow its arch foe to develop atomic weapons, something Tehran denies seeking.

By accepting an invitation from the Republican Party to address Congress on Tuesday, the Israeli leader infuriated the Obama administration, which said it was not told of the speech before plans were made public in an apparent breach of protocol.

A senior Israeli official told reporters on Netanyahu's flight that Congress could be "the last brake" for stopping a nuclear deal with Iran.

Saying it was Israel's impression that members of Congress "do not necessarily know the details of the deal coming together, which we do not see as a good deal," the official said Netanyahu in his speech would give a detailed explanation of his objections to an Iran deal.

U.S. Secretary of State John Kerry reiterated Washington's determination to pursue negotiations with Iran, saying on Sunday the United States deserved "the benefit of the doubt" to see if a nuclear deal could be reached.

Last week, Obama's national security adviser, Susan Rice, said the partisanship caused by Netanyahu's looming address was "destructive to the fabric of U.S.-Israeli ties".

But Netanyahu, who is running for re-election in a March 17 ballot, has framed his visit as being above politics and he portrayed himself on Sunday as being a guardian for all Jews.

"I'm going to Washington on a fateful, even historic, mission," he said as he boarded his plane in Tel Aviv.

"I feel that I am an emissary of all Israel's citizens, even those who do not agree with me, and of the entire Jewish people," he told reporters.

Some Democrats to boycott speech

U.S. officials fear he is seeking to sabotage the Iran diplomacy, and critics have suggested his visit is an elaborate election stunt that will play well with voters back home.

While White House and Israeli officials insist that key areas of co-operation from counterterrorism to intelligence to cyber security will remain unaffected, the deepening divide over the Iran talks is shaping up as the worst in decades.

U.S. Secretary of State John Kerry told the ABC program This Week that the U.S. deserves the "benefit of the doubt" to find out whether its partners in nuclear talks can reach a final agreement with Iran. (Laura Lean/Pool/Associated Press)

Previously Israel has always been careful to navigate between the Republican and Democrat camps. The planned address, however, has driven a rare wedge between Netanyahu's government and some congressional Democrats. Some two dozen or more of them plan to boycott the speech, according to unofficial estimates.

Kerry said on Sunday the United States deserves "the benefit of the doubt" to see if a nuclear deal can be reached.

Kerry, speaking on the ABC program This Week, also said he hoped Netanyahu's speech to Congress did not turn into "some great political football."

Undermining ties

Speaking in Tehran on Saturday, Iranian Foreign Minister Mohammad Javad Zarif accused Netanyahu of trying to undermine the nuclear talks in order to distract from the Palestinians' unresolved bid for an independent state.

"Netanyahu is opposed to any sort of solution," Zarif said.

Hard-line U.S. supporters of Israel say Netanyahu must take centre-stage in Washington to sound the alarm over the potential Iran deal, even at the risk of offending long-time supporters.

Netanyahu, as well as many in Israel, view a nuclear-armed Iran as a threat to its very existence, citing Tehran's repeated calls for Israel's destruction and its support for groups like Hezbollah. (Nir Elias/Reuters)

But a U.S. official, speaking on condition of anonymity, said the "politicized" nature of his visit threatened "what undergirds the strength of the relationship".

As one former U.S. official put it: "Sure, when Netanyahu calls the White House, Obama will answer. But how fast will he be about responding (to a crisis)?"

Last month, U.S. officials accused the Israeli government of leaking information to the Israeli media to undermine the Iran negotiations and said it would limit further sharing of sensitive details about the talks.

Israelis are fretting over the possibility that Washington might not be as diligent as before about shielding Israel at the United Nations and other international organizations.

Direct challenge to Obama

One Israeli official said this was becoming more worrisome as the Palestinians are resorting increasingly to global forums like the International Criminal Court to press their grievances.

Netanyahu is expected to use his speech to urge Congress to approve new sanctions against Iran despite Obama's pledge to veto such legislation because it would sabotage nuclear talks.

"What the prime minister is doing here is simply so egregious that it has a more lasting impact on that fundamental underlying relationship," said Jeremy Ben-Ami, head of J Street, a liberal pro-Israel lobbying group aligned with Obama's Iran policy.

Netanyahu, who will address the influential pro-Israel lobby AIPAC on Monday, has remained defiant. Even so, he is expected to try to keep tensions from spiralling.

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